I. Field of the Invention
The present invention relates to implanted orthopedic, maxillofacial, spinal and plastic surgery hardware. More specifically, the present invention relates to cushions which cover such hardware to cushion the surrounding tissue from such hardware.
II. Related Art
Orthopedic implants made of various materials are employed to provide adequate strength to resist biologic stress across a pathologic area during bone healing. Orthopedic implants are often made of metal such as titanium which can be forged or bent in a variety of ways to allow such implants to conform to a surface of the bone to some degree while maintaining the mechanical properties of the implant. Despite new advances in low profile implant technologies, metal implants are limited in their ability to match the contour of the underlying bone. The result is often a step-off at the bone-implant interface. The existence of such a step-off can cause a variety of issues. Such a step-off is often the site of discomfort for patients, particularly when skin, muscle, or other tissues are pushed against the step-off. In addition, the surfaces of metal implants are a source of friction between such implants and surrounding soft tissues which are constantly moving. Problems arising from such friction are most extreme when a tendon is overlying the implant. Such friction can not only cause painful tendonitis, but can also eventually lead to a rupture of the tendon.
Screws are sometimes used alone or to fasten plates, rods, and other orthopedic implants. Whether used alone or to fasten another orthopedic implant in place, the screws are often prominent and painful. For example, in spinal fusion surgery screws are typically placed low in the back where soft tissue coverage is thin and pressures from the waist line of clothing and belts or furniture when sitting is significant. The result is often painful irritation. In extreme cases, the implanted hardware can erode through the skin causing dangerous ulceration requiring prolonged wound care and increased risk of infection. All too often secondary surgeries are required to remove the hardware, reposition the hardware or graft skin to treat such an ulcer.
Issues of the type described above are not uniquely associated with orthopedic implants. Dental and maxillofacial implants used to treat or repair bones of the jaw and face and similar implants used in plastic surgery often result in a step-off at the bone-implant interface or in friction between the implant and soft tissue covering the implant. Such a step-off or friction can lead to substantial discomfort and in severe cases ulceration, infection, and damage to the surrounding tissue.
Various attempts have been made to address step-offs at the bone-implant interface of plates used to treat bone fractures. Most involve efforts to alter the contours of the implant to reduce the step-off. Pre-formed plastic covers with tapered edges have also been employed to cover such plates. Neither of these two approaches have successfully or universally solved the significant problems which exist. The significant limitations of these two approaches becomes apparent after a surgeon modifies the plate shape to fit the affected bone. Bones have significant individual differences in shape. Failing to properly modify the shape of the plate to fit the bone can cause poor fracture healing if the bone fragments are not properly opposed. When the surgeon modifies the shape of the hardware to provide proper opposition of the bone fragments, the prominence of the hardware and the bone-implant step-off often becomes exaggerated. Also, once the shape of the plate has been modified, preformed plastic covers often no longer fit the implant. The application of a preformed plastic cover after the surgeon modifies the shape of the plate so the bone fragments are properly opposed is often not possible or the result is an unacceptably loose, insecure fit between the plate and the cover. For such a cover to be used and provide any benefit it must not only be securely locked to the plate so it does not float within the body, but it must also reduce the bone-implant step-off. While such covers can be designed to fit a plate of a particular shape and reduce the step-off, any change in the shape of the plate can result in the cover exacerbating rather than reducing this step-off.
Various malleable polymers have been employed to fill bone voids and as bone graft alternatives. The use of such materials has broad application in the field of dentistry to fill voids in teeth. Such materials have also been used to act as a scaffolding for bone healing. These materials have different properties to allow for varying work times, stiffness, and strength. For a variety of reasons, such materials have rarely, if ever, been used to reduce friction between an implant and surrounding tissue or a step-off at the bone-implant interface.
Other bio-absorbable polymers have been employed in the design of internal bandages to allow for guided tissue regeneration or to prevent adhesion of tissue at a surgical site during healing. These materials have not been employed to either reduce friction or to reduce a bone-implant step-off. The use of a bio-absorbable material would likely only temporarily reduce friction. Also, generation of tissue around an implant could make explantation more difficult and give rise to a number of other anatomical issues making the use of such materials undesirable.
In view of the foregoing, there continues to exist a real, significant and long-felt need for an apparatus which successfully and nearly universally addresses the problems associated with a bone-implant step-off and at the same time reduces friction between the implant and surrounding tissue.